"Splicing" or joining of optical fibers is a well known and widely practiced technique in the field of fiber optics. Two of the most common methods for splicing standard 125 micron diameter fused silica optical fibers include thermal fusion, and the use of photo-cured adhesive materials. In thermal fusion, adjacent ends of the optical fibers which are to be joined are brought close to each other. The fibers are arranged to insure that the core of the optical fibers, typically having a diameter in the range of 4 microns, are aligned and are coaxial with each other. When the optical fibers are properly aligned, an electric arc is generated between two electrodes. The electrodes are positioned such that the electric arc travels directly between the two optical fibers. Heat generated by the electric arc creates a "melt zone" or uniform softened area of glass at the end of each of the optical fibers. The optical fibers are then pushed or forced towards each other such that the uniformly softened ends of the fibers contact each other. Thus, when the softened glass hardens, the two optical fibers are joined.
Although such a technique is suitable for many applications using standard 125 micron fused silica optical fibers, thermal fusion is not without significant drawbacks. For example, currently available thermal fusion devices are extremely expensive. Portable equipment used, for example by telephone companies, to thermally fuse optical fibers costs, on average, between 50,000 and 100,000 dollars per unit. Furthermore, such equipment is only suitable for standard 125 micron diameter optical fibers. That is, current thermal fusion devices will only splice standard 125 micron optical fibers.
Additionally, even if currently available thermal fusion devices were able to be modified to handle larger diameter optical fibers, the fundamental technique of thermal fusion is not well suited to larger diameter optical fibers. Specifically, as the core diameter of the optical fibers are increased, a much larger melt zone must be created. Consequently, substantially higher current would be necessary to generate sufficient heating. However, as the core diameter increases it is extremely difficult to achieve uniform heating of the end of each optical fiber. That is, thermal fusion techniques can not be easily controlled to generate a large and uniform melt zone.
As an additional drawback, thermal fusion techniques often result in deformation of the host fibers. That is, in thermal fusion the optical fibers to be joined are heated until the ends of the fibers are softened. The softened ends are then pushed or forced together. In so doing, often the ends of the optical fibers are warped or sprayed radially outward resulting in a slightly increased core diameter. This deformation causes light losses and reduces the efficiency of the spliced optical fiber.
Furthermore, the thermal cycling associated with heating and cooling of the optical fibers can cause brittleness to occur within the optical fibers. The introduction of brittleness is especially prevalent in non-fused silica fibers such as, for example, rare earth doped optical fibers.
In optical fiber splicing using photo-cured adhesives, typically a photo-curable plastic, such as a urethane or acrylate resin, is inserted into a capillary tube having a diameter just slightly larger than 125 microns. The optical fibers to be joined are inserted at opposite ends of the capillary tube and are pushed towards each other. The diameter of the capillary tube being only slightly larger than the diameter of the optical fibers insures that the 4 micron cores of the optical fibers are properly aligned. Additionally, because the diameter of the capillary tube is only slightly larger the diameter of the optical fibers, a vent or hole is located in the surface of the capillary tube to allow the escape of air or other gases as the two optical fibers are pushed towards each other.
The optical fibers are forced towards each other such that the photo-curable adhesive material is compressed into a thin layer between the ends of the two optical fibers. The excess photo-curable material is forced out of the vent hole in the capillary tube. The photo-curable material is then exposed to light, typically ultraviolet light to cause, the photo-curable material to harden and bond the optical fibers together.
Like thermal fusion, optical fiber splicing using photo-curable adhesives also has significant drawbacks. Specifically, the photo-curable adhesives are subject to significant thermal and photonic degradation. That is, as the adhesive material is subjected to heat and/or light, the transmissive ability and structural integrity of the adhesive material is reduced. In typical applications, a standard 125 micron fused silica optical fiber transmits power on the order of 1/100 of a watt. Even at such power levels, photonic and thermal degradation occurs. Thus, in applications where optical fibers having 600-1000 micron core diameters are transmitting power on the order of 100's of watts, photonic and thermal degradation are greatly accelerated. Furthermore, photo-curable adhesives also suffer from degradation as a result of aging even without significant exposure to light or heat. Thus, the use of photo-curable adhesives is limited to low power, low heat, and short term applications.
Therefore a need exists for an inexpensive technique to splice optical fibers which does not cause deformation of the host fibers, does not require repeated thermal cycling of the optical fibers, which is resistant to thermal and photonic degradation even at high power applications, which does not prematurely deteriorate with age, and which is suitable for use with optical fibers having a core diameter of as much as 1000 microns or greater.